First, the most likely cause is the starter clutch getting tired and worn. It is the over run clutch that lets the starter gear that engages the ring gear on the clutch basket spin faster than the starter as the engine catches. It is a sprag clutch that starts to make that noise in the early stages of failure. If you still have the factory compensator, it probably makes an occasional loud metallic bang or crack when you press the starter button. The bang is the compensator slamming against the max travel limit because of a weak spring. That bang puts a heavy shock load on the starter clutch and causes early failure. Mine went out at 15,000 miles. Sometimes a new battery "cures" the grind for a while, but it will return. Unfortunately the only long term fix is to replace the compensator with the latest Harley version or an after market different design. Baker has one that most like pretty well but it is more expensive than the Harley compensator. Both will require a new rotor, the current one is not compatible with either new compensator. Finally the list is completed with a replacement starter drive. "Allballs" makes a popular one which has worked well for me about 8 years. You are far from the first person with an 07 0r 08 96 ci engine who has needed to change these parts. Few others have gotten 50,000 on the factory equipment as you have. There are many pictorials and videos on net for the compensator change if you are one who likes to do his own work. If not, you will at least see what the old and new parts look like. Harley didn't upgrade the compensators from the factory until the '11 model year and that early version comp has its own set of problems. The latest version seems to be doing pretty well and that is all that Harley sells now. If you hear a "growl" as the engine comes to a stop at shut down, you probably need a new inner primary bearing also. I believe that the inner primary bearing is also damaged by the compensator slamming against its stop. Wish it was better, less expensive news, but many have had the symptoms you describe and the above is the fix.
